Conventionally, there are known sensor-detachable illumination devices in which a sensor is detachably attached to a device housing (see, e.g., Japanese Patent Laid-open Publication No. 2002-110376). FIG. 25 schematically illustrates a conventional illumination device 40 as obliquely seen from below. The illumination device 40 is fixed to a ceiling or the like and includes an illumination lamp 41 such as a fluorescent tube, a sensor 42 for use in controlling the lighting operation of the illumination lamp 41 and a sensor attachment unit 44 through which the sensor 42 is attached to a device housing 43. The sensor 42 includes, e.g., a sensing unit 42a for detecting the presence of a person and a sensing unit 42b for detecting the brightness. The sensor 42 is attached to the sensor attachment unit 44 so that the sensing units 42a and 42b can be exposed to the outside.
FIGS. 26A and 26B show the external appearance of the sensor attachment unit 44 employed in the illumination device 40. FIG. 26A illustrates a state that the sensor 42 is not attached to the sensor attachment unit 44, while FIG. 26B illustrates a state that the sensor 42 is attached to the sensor attachment unit 44. As can be seen in FIGS. 26A and 26B, the illumination device 40 undergoes a remarkable change in outward appearance in the vicinity of the sensor attachment unit 44 depending on the attachment and detachment of the sensor 42. If the sensor 42 remains attached at all times, the illumination device 40 will undergoes no change in outward appearance and therefore will pose no problem in design. However, if the illumination device 40 is of the type capable of exercising a specified function without resort to the sensor 42, a problem in design is posed in that the outward appearance of the illumination device 40 varies depending on the presence or absence of the sensor 42. In other words, the sensor attachment unit 44 of the illumination device 40 is exposed in case where the sensor 42 is not attached in place. This leaves a conspicuous difference in outward appearance between the illumination device 40 without the sensor and another illumination device with a sensor. In case where an illuminating system includes a plurality of illumination devices 40, the system lacks uniformity in outward appearance and suffers from a big problem in design if the respective illumination devices 40 differ in outward appearance from one another. As a separate matter, the sensor 42 and the sensor attachment unit 44 are electrically connected to each other by a connector 44c or the like. The connector 44c remains exposed when the sensor 42 is not attached to the sensor attachment unit 44. In this state, the charging terminal of the connector 44c is exposed to the outside, meaning that a user may inadvertently touch the charging terminal of the connector 44c. This is undesirable from the standpoint of product quality and safety.